This invention relates in general to digital filters. More specifically, this invention relates to digital filters utilized for tone encoding and tone detecting in communications systems.
Digital frequency sensing circuits and filters are being widely used to replace their analog counterparts because of their relatively small size, high temperature stability, and uniformity in mass production. These characteristics are particularly desirable when frequency sensing circuits are used for detecting relatively low frequencies such as audio frequencies. Analog circuits require the use of relatively large inductors and capacitors at such frequencies. Furthermore, relatively large analog component are not usually manufactured within tolerances required and must therefore be individually tested and adjusted, a procedure that is expensive. In addition, large analog components are subject to an unacceptable level of temperature drift.